Fumer la moquette

Literally: To smoke the carpet

Example: “Il pense ouvrir un restaurant sur la Lune ? Il fume la moquette !” — He's thinking of opening a restaurant on the moon? What has he been smoking?

English equivalent: What have you been smoking?

Origin: Second half of the 20th century, and mockery aimed at smokers. Carpet pile is the closest thing to grass a city flat has to offer — visually, at least — so the joke assembles itself: a man whose supply has run out crops a handful of moquette into his joint and sets off on an unusual trip. Anyone smoking synthetic carpet fiber is plainly not in possession of their faculties, which is what the phrase says about anyone talking rubbish. There is a footnote in its favor: hashish is Indian hemp, and hemp was for centuries the fiber of choice for rope, cloth and carpets, so at home the distance between fumer le tapis and fumer la moquette is shorter than it sounds.